Morphine is the drug of choice when treating short-term moderate to severe pain. However, prolonged use often leads to the development of tolerance and eventually dependence. The neural mechanisms of these phenotypes are not fully understood. Acutely, morphine takes action on opioid receptors in the brain resulting in inhibition of the neurons containing the receptors. In the midbrain, agonist binding to opioid receptors on GABA neurons results in disinhibition of dopamine (DA) neurons. The resulting increase in DA release is thought to contribute to the rewarding and addicting aspects of opioids. Recent evidence indicates that there are several opioid-sensitive GABA inputs to midbrain DA neurons. One such GABAergic projection originates from medium spiny neurons (MSNs) in the nucleus accumbens (NAc) and projects to the ventral tegmental area (VTA). Preliminary results indicate that both D1 and D2 DA receptor-expressing MSNs project to the VTA. These two sets of neurons express different opioid receptors (kappa and mu respectively) and synapse onto distinct target neurons. Additionally, the NAc projections target both DA and GABA neurons in the VTA. Previous research has shown that distinct opioid-sensitive inputs show different sensitivities to opioids as well as different levels of tolerance and withdrawal following chronic opioid exposure. The overall goal of this project is to characterize the two distinct NAc afferents to the VTA and discover how they change in response to acute and prolonged opioid exposure. The overall hypothesis is that the relative roles of the NAc afferent subsets will change following chronic morphine treatment. The specific aims of this proposal are as follows: (1) to identify the target neurons and target receptors in the VTA that receive input from each of the NAc projections, (2) to identify chronic morphine-induced adaptations in each of the NAc projections to the VTA, and (3) to determine the adaptations that occur in the GABAB-GIRK signaling complex in response to chronic morphine. Together, these aims will elucidate the role of the opioid-sensitive neural projection of GABAergic neurons in the NAc to the VTA in morphine dependence.